The invention relates to a method for controlling the dryer section of a paper machine. The invention also relates to an apparatus for controlling the dryer section of a paper machine.
The prior art includes systems in which the temperature of drying cylinders is determined and problems in the dryer section are detected on the basis of the temperature. For example, U.S. Pat. No. 5,746,009 discloses a process and apparatus for controlling the surface temperature of a drying cylinder of a paper machine. The cylinder surface temperature is determined and controlled by using measurements of air-steam and condensate mixture temperatures emerged from the drying cylinder. Thus, the measurements do not give the actual temperature of the cylinder surface, but the temperature of the media inside the cylinder. These temperatures are not equal. Consequently, this method does not give true and accurate values for drying cylinder control.
The object of the present invention is to provide a method and apparatus by means of which a dryer section can be monitored and controlled accurately and reliably.
The basic idea of the invention is that the basis weight and moisture of a paper web, the energy fed into the cylinders of the drying apparatus and the machine speed are measured, and then the surface temperature of the drying cylinders of a paper machine is predicted on the basis of the measured values. At the same time, the actual surface temperature of the cylinders is also measured, and if the measured value deviates from the predicted value, any defects in the drying apparatus can be detected quickly. Another idea of the invention is to determine the normal correlation between the power consumption of the drive motors of the cylinders in the drying apparatus and the machine speed and to measure these variables. If the correlation of the variables is deviant, it can be concluded that there is a failure in the drying apparatus. In that case it is possible to identify the dryer cylinder where the failure is by measuring the temperature of the cylinders. This can be determined on the basis of the fact that the dryer cylinder which for example cools down faster than the others is the cylinder where the failure is.
An advantage of the invention is that the control and maintenance of the dryer section can be improved and the quality and production enhanced since the problems occurring in the drying apparatus can be identified quickly and accurately. Further advantages include improved temperature control in ramp up sections, faster grade changes, fewer breaks and faster recovery from sheet breaks. Furthermore, thanks to the better monitoring of the dryer section and real-time information, curl cockle, mettle and shrinkage can be reduced, and printing surface as well as other properties of paper can be improved.
Thus, in accordance with one aspect of the present invention, a method for controlling the dryer section of a paper machine is provided, the method comprising measuring the basis weight and the moisture of a fiber web, the drying capacity factor of the dryer section and the machine speed, determining the surface temperature of a drying cylinder of the dryer section on the basis of these measurements, measuring the surface temperature of the drying cylinder of the dryer section and comparing the determined value with the measured value and determining on the basis of deviation between the values whether the dryer section functions correctly.
The apparatus of the invention comprises means for measuring the basis weight and moisture of a fiber web, means for measuring the machine speed, a control unit for determining the surface temperature of a drying cylinder on the basis of these measurements and means for measuring the surface temperature of the drying cylinder, the control unit being arranged to compare the determined value with the measured value and determine on the basis of deviation between the values whether the dryer section functions correctly.
In this application, the term xe2x80x9cpaperxe2x80x9d also refers to paper board, tissue and sheet of pulp.